Mayor de Blasio’s top money man funneled $50,000 of “publicly untraceable” campaign cash to help a Long Island Democrat win a crucial race for the state Senate, according to a report Monday night.

Ross Offinger, de Blasio’s campaign finance director and treasurer of his now-shut advocacy group Campaign for One New York, was behind the windfall, which he donated through a shadowy Delaware corporation, according to the Albany Times-Union.

Ross OffingerGregory P. Mango

Offinger, already under scrutiny for fund-raising activities involving de Blasio, sent the money to the Nassau County Democratic Committee, which was backing Todd Kaminsky, the Times Union said.

Kaminsky edged out Republican Chris McGrath by 780 votes in the April 19 special election to fill the seat formerly held by convicted Republican Senate Majority Leader Dean Skelos.

The funds were wired to the committee’s account by a Delaware company named A&J Contracting, the paper said, so there was no check signature or paper trail.

That company, however, has no phone number or Web site. It also had not made a donation to a political party or candidate before the $50,000 gift.

A spokesman for the Nassau Democratic Party confirmed to the Times-Union that the money was actually raised by Offinger.

It was not clear how Offinger — who was paid $14,000 by the Nassau Democrats for fund-raising — garnered the money.

“I really don’t have a comment on specific donors, but I’m proud of the work we did in Nassau County,” Offinger told the paper.

Offinger is currently being probed by US Attorney Preet Bharara in a widening investigation into campaign financing.

De Blasio, meanwhile, is being investigated along with his top aides for “willful and flagrant” violations of campaign-finance law. It has been alleged that the mayor steered $1 million in union and corporate donations to three upstate Democrats running for state Senate seats in 2014.